In recent years, mobile communication systems or IP telephone systems, for example, in which plural communication networks are selectively used for communication, are realized.
In such communication systems, a call controller determines a communication network adaptable to a communication at a terminal device on the receiving side. In this determination, however, it is difficult to determine the communication network depending on information already held by the call controller.
This is because it is difficult to always monitor all conditions of the communication networks relating to the terminal device on the receiving side.
For this reason, a reception process is sequentially tried through plural communication networks estimated to be available, based upon the information on the conditions of the communication networks regularly updated. As a result, generally, a communication network to which the terminal device on the receiving side has made a normal response is determined to be a network for use in call setting.
Accordingly, as the number of the communication networks available to the terminal device on the receiving side is increased, there is a tendency of increasing the average period of time for determining the communication network to be used for the call setting.
On the other hand, there is a proposed technique of selecting the communication network by the user's determination or automatic determination made by a server, in selectively applying plural communication networks for a call connection between the terminal device on the transmitting side and the terminal device on the receiving side (for example, see Patent Document 1).